The United States Air Force has announced that it will launch a secret space plane that has sparked speculation about the militarization of space.

The Pentagon has set April 21 as the date for the launch of the robotic space plane known as the X-37B Orbital Test Vehicle (OTV), which is a reusable unmanned plane capable of long outer space missions at low orbits.

Since the nature of the project is shrouded in mystery, defense analysts allege that the US military is building the first generation of US 'space Predator drones' that will build up the United States' space armada, the Christian Science Monitor wrote in a recent article.

Military experts argue that the US Department of Defense would not have saved NASA's costly X-37B project, which had been scrapped, if it did not have a military application.

They say the US wants to maintain a leading role in space via the development of the new 'space weapon' at a time when other countries like China are expanding their space programs.

However, US military officials maintain that the X-37B will only be used for transporting payloads and facilitating space experiments.

The OTV is capable of supporting a range of tests, the Air Force spokesperson for the project said earlier at the 26th National Space Symposium.

"The first mission will emphasize proving technologies necessary for long duration reusable space vehicles with autonomous reentry and landing capabilities," Angie Blair added.

She went on to say that the "specific details of the OTV capabilities, limitations and vulnerabilities" remain classified.

The X-37B can stay at an orbit between 200 and 800 kilometers for around 270 days before landing automatically at Vandenberg Air Force Base in California, reports say.

The location of the mission control center for the Boeing-made space vehicle is also a classified secret, but Blair says that Air Force Space Command's 3rd Space Experimentation Squadron (AFSPC) will run the operation.

Military space specialist Professor Roger Handberg, who is the chair of the Department of Political Science at the University of Central Florida in Orlando, told Space.com that the X-37B project may signify continued U.S. Air Force interest in a rapid response vehicle along the lines of the long-proposed space maneuver vehicle.

He added that the project could be viewed "as the logical extension of the push into unmanned aerial vehicles (UAVs) where vehicles used for observation have moved into weapon carriers and various other missions, many classified."

"From the perspective of international observers, especially in space-aspiring states such as China, the X-37B program just reinforces their view that the U.S. is pushing to gain first mover advantage in rapid response, including possible weaponization of space using this vehicle or a derivative," Handberg noted.

Political analysts say that the X-37B project could be interpreted as a violation of the Outer Space Treaty of 1967 if the space plane is used for military purposes.

The Outer Space Treaty of 1967, officially known as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, states that the exploration and use of outer space shall be carried out for the benefit and in the interests of all countries and shall be the province of all mankind; states shall not place nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies or station them in outer space in any other manner; the Moon and other celestial bodies shall be used exclusively for peaceful purposes; astronauts shall be regarded as the envoys of mankind; states shall be liable for damage caused by their space objects; and states shall avoid harmful contamination of space and celestial bodies.

Article IX of the Outer Space Treaty states: "A State Party to the Treaty which has reason to believe that an activity or experiment planned by another State Party in outer space, including the Moon and other celestial bodies, would cause potentially harmful interference with activities in the peaceful exploration and use of outer space, including the Moon and other celestial bodies, may request consultation concerning the activity or experiment."

In addition, a proposal has been put forward for a Space Preservation Treaty that would ban all space weapons, but no country has signed the treaty so far.